The present invention relates to a bus bar for supplying a predetermined power voltage to a circuit board, particularly to a structure allowing to the direct securing of a relatively thick and long bus bar to a circuit board.
A circuit board package in certain electronic equipment, such as middle- or large-scale computers, for instance, requires a supply of a large current of a predetermined voltage. The current sometimes amounts to a few or several hundred amperes. A bus is provided to constitute a power supply line for components such as ICs of an electronic circuitry formed on the circuit board. Generally, the larger the size of a circuit board, the greater the current to be supplied to the circuit board. Further, a large-sized circuit board needs a long bus. Accordingly, a bus to be associated with a large-sized circuit board is required to have a large cross-sectional area; for example, a cross-sectional area of 100 to 200 mm.sup.2 is necessary for a bus supplying a circuit board having a side length of 50 cm with a current as mentioned above. Such a bus usually has a bar-like structure and is referred to as a bus bar.
There are various disclosures relating to buses for supplying power voltages, including those classified into two categories: 1) a relatively thin bus formed from a conductor plate (Tokukaisho 54-68972 and Tokukaisho 55-18493), the conductor plate being directly secured to a corresponding circuit board by using screws or soldered to the pins of a connector mounted on the board; 2) a relatively thick bus having a bar structure, which is usually mounted on a mother-board (or back-board) and is provided with a connection to a corresponding circuit board package through a specially designed connector (Tokukaisho 59-44910).
As is evident from these disclosures, a relatively thin bus, which is associated with a relatively small-sized circuit board requiring a supply of a relatively small current, can be secured to the circuit board by soldering or by screws. On the other hand, for a relatively large-sized circuit board requiring a supply of a relatively large current and unassociated relatively thick bus, it is difficult to attain a uniform solder joint between the bus and the board because of the circuit inherent warping thereof. In the securing of a large-sized circuit board by means of screws, stresses are produced in the circuit board because the inherently warped circuit board is forced so as to come in close contact with the bus bar. These stresses often cause a defects, such as cracks, in the circuit board or the electronic components mounted thereon.
A relatively thick bus, i.e. bus bar, which is secured to a circuit board is disclosed in Tokukaisho 59-104192. This bus bar is intended to reform the inherent warp of a circuit board, reduce the impedance of a power supplying pattern formed on the board, and supply a power voltage therethrough to the circuit on the board. The disclosed bus bar is supported by conductor pins secured to the circuit board.
A circuit board is usually subjected to heat cycles from room temperature to around 100.degree. C. or higher due to the heat dissipation caused by a large current as mentioned before, which is supplied for the operation of the circuit board package. Accordingly, the circuit board is subject to additional stresses caused by the differential thermal expansion between the bus and the circuit board during the heat cycles. These stresses enhance the danger of the formation of defects in the circuit board or the components mounted thereon. Defects due to the differential thermal expansion are complicated by the materials of the bus bar and the circuit board, and by the temperature distribution on the board. Thermal stresses impose a more serious problem for circuit boards composed of a ceramic, e.g., alumina, because of the greater difference between the thermal expansion of alumina and that of copper which is usually used for a bus bar, and also because of the low flexibility of alumina.
The prior art conductor pins for securing a thick bus bar to a circuit board are effective for absorbing the stress exerted on the circuit board; however, it is undesirable, from the view points of process simplicity and manufacturing cost, to incorporate such conductor pins. Further, it is advantageous, for the uniform supply of a power voltage of a large current throughout a circuit board, to secure a bus bar so that it is in direct contact with a power supplying pattern on a circuit board. Therefore, it is desirable to provide a bus bar having a structure which allows the bus bar to be directly secured to a circuit board and which prevents the bus bar from producing defects in the circuit board or the electronic components mounted on the board.